Restore the sqrt -> llvm.sqrt mapping in fast-math mode

This restores the sqrt -> llvm.sqrt mapping, but only in fast-math mode
(specifically, when the UnsafeFPMath or NoNaNsFPMath CodeGen options are
enabled). The @llvm.sqrt* intrinsics have slightly different semantics from the
libm call, specifically, they are undefined when given a non-zero negative
number (the libm calls will always return NaN for any negative number).

This mapping was removed in r100613, and replaced with a TODO, but at that time
the fast-math flags were not yet implemented. Now that we have these, restoring
this mapping is important because it will enable autovectorization of sqrt
calls in loops (at least in fast-math mode).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190646 91177308-0d34-0410-b5e6-96231b3b80d8